<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        * {
            margin: 0;
            padding: 0;
        }
        
        body {
            width: 100%;
            height: 500px;
            display: flex;
            flex-direction: row;
            flex-wrap: wrap;
        }
        
        body>div {
            flex: 1 0 50%;
        }
        
        body>div:first-child {
            flex: 0 1 100%;
        }
        
        .item1 {
            background-color: red;
        }
        
        .item2 {
            background-color: blue;
        }
        
        .item3 {
            background-color: green;
        }
    </style>
</head>

<body>
    <div class="item1">

    </div>
    <div class="item2">

    </div>
    <div class="item3">

    </div>
</body>

</html>